Mote study sheds light on how sharks feed

Published: Wednesday, April 2, 2014 at 5:01 p.m.

Last Modified: Wednesday, April 2, 2014 at 5:01 p.m.

SARASOTA - Citing a Hollywood movie as reference material can be like relying on Aunt Jemima Lite to lose calories.

But every now and then, even big-screen hype can't help but get it right.

“What we are dealing with here is a perfect engine, an eating machine,” Richard Dreyfuss' biologist declared in the clownish horror of “Jaws” in 1975. “It's really a miracle of evolution. All this machine does is swim and eat and make little sharks, and that's all.”

That was nearly 40 years ago, before an unprecedented and just-released study took the first hard look at the skills of some of nature's most efficient aquatic hunters.

In the science journal PLOS ONE released Wednesday, researchers from Mote Marine, the University of South Florida and Boston University ran three species of sharks through rigorous sensory-deprivation tests to assess their ability to compensate for the imposed loss of targeting triggers, such as sight and smell.

“Our results demonstrate that sharks are capable of attending to multiple sensory cues simultaneously, switching sensory modalities ... as they approach their prey,” the report states. “Their flexibility in behavior suggests that sharks are well adapted to succeed, even in the face of a changing environment and evolutionary advancements in prey defenses, including chemical, visual, and mechanical camouflage.”

The six-year study, enabled by a $500,000 grant from the National Science Foundation, involved jamming up one or more of the carnivores' hunting modes, which include sight, touch, smell, lateral lines and electroreception.

Lateral lines are sense organs fish use to identify movement and vibration in the water; electroreception is the ability to detect living-prey signals such as heartbeat and respiratory function.

Three species — nurse sharks, blacktips and bonnetheads — were selected for comparison of their distinct and separate feeding patterns.

Using multiple and innovative combinations of sensory blocking, scientists documented significant variations in a “hierarchy” of hunting responses.

“We found out that blacktips and bonnetheads don't need smell at all,” says Gardiner, who was with the University of South Florida at the time. “But a nurse shark won't try to capture something without smell.”

A nocturnal hunter, the nurse shark doesn't rely on eyesight to find its prey, and relies largely on contact to guide its suction biting.

Blacktips and bonnetheads, on the other hand, can forage when deprived of either smell or sight, but not both simultaneously.

Soft-tissue fossil records are scant for these 400 million-year-old cartilagenous animals, but the Mote analyses would appear to bode well for their survivability in modern environments increasingly degraded by pollutants, runoff and turbidity.

Gardiner says multisensory studies were first discussed in the 1980s, but the logistical challenges were daunting.

Among the obstacles was how to design the best “plume-flow” tanks to simulate natural ocean conditions. Then came the dilemma of how to block sharks' senses with minimally invasive means.

“There's a lot of cost associated with these animals and anything we did with them had to be temporary and reversible because we wanted to use them for more than one task,” Gardiner says.

“We actually investigated the possibility of contact lenses for the sharks. In the old days, you probably would've had to put their eyes out if you wanted to conduct these sorts of tests. But that's not what we wanted to do. We wanted this to be as natural as possible with minimal stress.”

Ultimately, the team settled on covering the eyes with small pieces of heavy black plastic. Olfactory passages were sealed with cotton soaked in petroleum-based jelly. Other multi-sensory blocking ingredients included glue and anesthesia.

<p><em>SARASOTA</em> - Citing a Hollywood movie as reference material can be like relying on Aunt Jemima Lite to lose calories.</p><p>But every now and then, even big-screen hype can't help but get it right.</p><p>“What we are dealing with here is a perfect engine, an eating machine,” Richard Dreyfuss' biologist declared in the clownish horror of “Jaws” in 1975. “It's really a miracle of evolution. All this machine does is swim and eat and make little sharks, and that's all.”</p><p>That was nearly 40 years ago, before an unprecedented and just-released study took the first hard look at the skills of some of nature's most efficient aquatic hunters.</p><p>The biggest surprise from the Mote Marine Laboratory experiments?</p><p>It's not just about smell.</p><p>“They've talked forever about sharks being swimming noses,” says Jayne Gardiner, a post-doctoral fellow at Mote, referring to papers dating back to the 19th century. “But we've documented some incredibly flexible feeding behaviors.”</p><p>In the science journal PLOS ONE released Wednesday, researchers from Mote Marine, the University of South Florida and Boston University ran three species of sharks through rigorous sensory-deprivation tests to assess their ability to compensate for the imposed loss of targeting triggers, such as sight and smell.</p><p>“Our results demonstrate that sharks are capable of attending to multiple sensory cues simultaneously, switching sensory modalities ... as they approach their prey,” the report states. “Their flexibility in behavior suggests that sharks are well adapted to succeed, even in the face of a changing environment and evolutionary advancements in prey defenses, including chemical, visual, and mechanical camouflage.”</p><p>The six-year study, enabled by a $500,000 grant from the National Science Foundation, involved jamming up one or more of the carnivores' hunting modes, which include sight, touch, smell, lateral lines and electroreception.</p><p>Lateral lines are sense organs fish use to identify movement and vibration in the water; electroreception is the ability to detect living-prey signals such as heartbeat and respiratory function.</p><p>Three species — nurse sharks, blacktips and bonnetheads — were selected for comparison of their distinct and separate feeding patterns.</p><p>Using multiple and innovative combinations of sensory blocking, scientists documented significant variations in a “hierarchy” of hunting responses.</p><p>“We found out that blacktips and bonnetheads don't need smell at all,” says Gardiner, who was with the University of South Florida at the time. “But a nurse shark won't try to capture something without smell.”</p><p>A nocturnal hunter, the nurse shark doesn't rely on eyesight to find its prey, and relies largely on contact to guide its suction biting.</p><p>Blacktips and bonnetheads, on the other hand, can forage when deprived of either smell or sight, but not both simultaneously.</p><p>Soft-tissue fossil records are scant for these 400 million-year-old cartilagenous animals, but the Mote analyses would appear to bode well for their survivability in modern environments increasingly degraded by pollutants, runoff and turbidity.</p><p>Gardiner says multisensory studies were first discussed in the 1980s, but the logistical challenges were daunting. </p><p>Among the obstacles was how to design the best “plume-flow” tanks to simulate natural ocean conditions. Then came the dilemma of how to block sharks' senses with minimally invasive means.</p><p>“There's a lot of cost associated with these animals and anything we did with them had to be temporary and reversible because we wanted to use them for more than one task,” Gardiner says.</p><p>“We actually investigated the possibility of contact lenses for the sharks. In the old days, you probably would've had to put their eyes out if you wanted to conduct these sorts of tests. But that's not what we wanted to do. We wanted this to be as natural as possible with minimal stress.”</p><p>Ultimately, the team settled on covering the eyes with small pieces of heavy black plastic. Olfactory passages were sealed with cotton soaked in petroleum-based jelly. Other multi-sensory blocking ingredients included glue and anesthesia.</p><p>Eighteen blacktips, 10 juvenile nurse sharks, and 16 bonnetheads participated in the study.</p><p>“So much of this work had to be conducted on the front end, trying to figure things out,” Gardiner says. “This was no small undertaking. Fortunately we had a great team.”</p><p>The team included Mote senior scientist Bob Hueter, Boston University biologist Jelle Atema, and USF's Philip Motta.</p><p>Although the analyses of sharks' adaptive mechanisms are encouraging, Gardiner cautions those skills pose no match for predators at the top of the food chain.</p><p>“The worst thing they're facing is overfishing,” she says. “That's not something their flexibility is going to help them with.”</p><p><empty></p>